A fix appears to have been released for Ubuntu 17.04, but that version won't install due to unmet dependencies that can't be satisfied with Armbian (libstdc++6 (>= 6)). The last stable build I could find for armhf is Firefox 52. You can download it from here via FTP:
These are the steps I took to remove the broken version, install the unmet dependencies, install the older, working version, and prevent it from being automatically upgraded back to the new, broken version.
sudo apt remove firefox
sudo apt-get install libpango1.0-0
sudo dpkg -i ~/Downloads/firefox_52.0.2+build1-0ubuntu0.12.04.1_armhf.deb
sudo apt-mark hold firefox
(TIL: gdebi is a very useful tool for seeing the dependencies of a package before you install it. You can use it from the command line like so:
sudo gdebi <name of deb package>
to see the dependencies you might need to install before installing a package.)
